In the field of graphic arts it is extremely useful that polymers having utility as, for example, printing plates be form-stable, adherent, abrasion resistant and water dispersible so that when these polymers are imaged the unexposed areas may be water or water/alcohol developable. In addition, it is desirable that these polymers photo-cure by crosslinking to a nontacky state in the presence of air. Such crosslinking often involves free-radical polymerization.
Historically, radical crosslinking of polymers has been effected by incorporating a high proportion (typically 50% by weight) of a polyethylenically unsaturated crosslinking agent in the polymer formation. Upon application of light or heat to generate free radicals, the polymer becomes immobilized by entrapment in the crosslinked network derived from a crosslinking agent or via chemical bonding. An alternative approach involves direct attachment to the polymer of photosensitive functionality, usually ethylenic unsaturation. This approach utilizes only one component and alleviates many problems associated with two-component systems, such as incompatibility, phase separation, etc. A disadvantage to this approach is that polymers of molecular weight in excess of 50,000 generally fail to effectively cure to tightly crosslinked networks. Consequently, desirable properties such as solvent resistance and abrasion resistance cannot be obtained. The presumed reason for the inefficient curing of polymers with molecular weight greater than 50,000 is that the higher molecular weight polymers have insufficient mobility (especially after several crosslinks have formed) for significant crosslinking interactions to occur.
Lower molecular weight polymers, i.e. those of molecular weight equal to or less than 5,000 readily cure to give high crosslink densities and subsequently good solvent resistance and abrasion resistance. However, these polymers lack inherent toughness and flexibility and consequently are brittle and lack tear resistance.
The incorporation of heterocyclic units into polymers including polyaldehydes, polyesters, polyamides, polyurethanes and polyethers in order to introduce hydrophilic properties into the polymers is described in the patent art. Examples of such art include U.S. Pat. Nos. 3,649,597 (polyaldehydes), 3,928,298 (polyethers), 3,893,979 (polyetherurethanes), 3,925,310 (polyamides), 3,928,289, 4,053,441 and 4,127,565. None of this art, however, teaches water-dispersible radiation-curable hydantoin unit-containing polyesters.
Water-soluble photocurable polyacrylates containing hydantoin as a heterocyclic unit are described in U.S. Pat. No. 3,808,226. These polyacrylates are prepared by condensation of unsymmetrical diglycidyl hydantoins with dicarboxylic acids to produce oligomers having a molecular weight of less than about 1400 that are then endcapped with acrylate functionality. Such heterocyclic unit-containing polyacrylates have only two acrylic groups per molecule. Also, their polymerization by exposure to radiation is relatively slow.
U.S. Pat. Nos. 3,920,732 (G.B. Pat. No. 1,400,978 is an equivalent), 3,804,735 and its division 3,856,744 (G.B. Pat. No. 1,400,979 is an equivalent), disclose photocurable compositions comprising mixtures of a beta-hydroxy ester and a polyitaconate. The beta-hydroxy ester is prepared by the reaction of an aliphatic aromatic or heterocyclic polyepoxide and ethylenically-unsaturated monocarboxylic acid. The polyitaconate is prepared by the reaction of about 0.8 to about 1 mole itaconic acid with about 1 to about 1.2 mole polyepoxide (preparation described in U.S. Pat. No. 3,847,770). These patents are directed to the production of solvent-free fluid polyester compositions that are not water-soluble and can be cured to a hard polymer. There is no disclosure of form-stable water-dispersible polyesters that can be cured by radiation.
U.S. Pat. No. 4,108,803 discloses water-soluble photopolymerizable epoxy resins containing pendant unsaturated ester or amidomethyl groups. Included among the epoxy resins disclosed in this patent are resins prepared by the reaction in a first stage of a heterocyclic diepoxide and dicarboxylic acids followed by reaction in a second stage of the product of the first stage with an unsaturated compound (for example, acrylic acid). This second stage reaction produces a polyester having a plurality of ethylenically unsaturated groups but does not produce polyesters having a plurality of backbone alpha-methylene groups.
The heterocyclic group-containing polyesters of this invention display the hydrophilic properties of hydantoin polymers described in the prior art. However, the polymers of this invention further incorporate photoactive functionality into the backbone of the polymer as opposed to pendant functionality described in the existing art. The procedure for incorporating photoactive backbone sites into the polymer offers an easier synthetic approach and provides greater crosslink density without sacrificing hydrophilicity than is attainable via the "pendant" approach. The high crosslink density available from the "backbone" approach provides cured films with enhanced resistance to abrasion and solvents. In addition to necessitating a second synthetic step to impart photoactivity to the polymer, the "pendant" approach generally reduces hydrohilicity by replacing a water-solubilizing functionality with an hydrophobic photoactive group.
The heterocyclic group-containing polymers of this invention are different from the polyepoxideitaconates described in the prior art. The prior art specifies the use of aromatic or aliphatic epoxides for the preparation of polyitaconates; consequently, these materials display no water solubility. This invention teaches the use of water-soluble heterocyclic epoxides which impart hydrophilic character to the polyitaconates. This property provides the ability to image these polymers and develop unexposed areas with water or water/alcohol developers. Furthermore, the polymers of this invention are form-stable, have enhanced adhesion to a variety of substrates and photocure readily to a nontacky state in the presence of air.
One aspect of the present invention relates to water-dispersible polyesters which photocure to a nontacky state in the presence of air. The backbone structures of these novel polyesters contain hydantoin units and alpha-methylene groups.
More particularly, the present invention relates to a family of water-dispersible, form-stable polymers of intermediate molecular weight, (i.e., 3,000-30,000) which cure rapidly in the presence of air to highly crosslinked networks that are solvent resistant, tough, and abrasion resistant.
Another aspect of the present invention relates to a method of preparing these novel polyesters.
A further aspect of this invention relates to radiation-sensitive, aqueous or aqueous/alcohol developable imageable elements containing the compositions disclosed herein.
An additional aspect of the present invention relates to the solvent resistant energy crosslinked compositions produced when the novel polyesters disclosed herein are cured.